This invention relates to an electronic spark timing advancing apparatus for controlling the ignition timing of an internal combustion engine, or more in particular to an electronic spark timing advancing apparatus using a microprocessor chip for calculating ignition timing.
Typical example of conventional electronic spark timing advancing apparatuses of this type is disclosed in an article entitled "GM Lays the Cornerstone with MISAR", AUTOMOTIVE INDUSTRIES, Nov. 1, 1976, Pages 29 to 32, in which the microprocessor chip takes informations of crankshaft position, engine speed, coolant temperature and intake manifold vacuum measured by several sensors and calculate the optimum spark advance of the conditions encountered referring to the programmed memory. Other apparatuses of such type in use include a mere wired logic system and an analog system. For detecting the engine load conditions, these systems use a throttle sensor for detecting the opening degree (angle) of a throttle valve, an intake manifold vacuum sensor for detecting the vacuum of the intake manifold, or an air flowmeter for detecting the flow rate of air flowing into the carbureter from the air cleaner. In these sensors, zero adjustment is accomplished by adjusting the mechanical position of the indicator of the sensor which is interlinked with, for instance, the throttle valve. In view of difficulty of precisely adjusting the mechanical position of the indicator of the sensor, complete zero adjustment of the indicator of the sensor is impossible and an error of the output of the sensor is unavoidable.
As a sensor for detecting the opening degree of the throttle valve, for instance, assume a potentiometer with an indicator (slider) interlinked with the throttle valve, in which the opening degree of the throttle valve is indicated by the output voltage (electric potential of the slider) of the potentiometer. The slider position at which the opening degree of the throttle valve is zero (this slider position is hereinafter referred as an actual zero position) is required to be adjusted in such a manner that the output voltage of the sensor is at a predetermined value, for instance, zero. The slider position under this condition is hereinafter referred as an expected zero position. This zero adjustment is so difficult and low in accuracy that an output voltage of the sensor contains error which cannot be neglected in calculating the ignition timing. Even if the zero adjustment is accomplished very accurately, the actual zero position of the slider develops an error and is displaced from the expected zero position slowly with time, with the result that the output voltage of the sensor comes to fail to represent an accurate opening degree of the throttle valve. An electronic spark timing advancing apparatus for advancing the spark timing on the basis of the output value of the sensor, therefore, is incapable of optimum control. Further, the zero adjustment of the sensor is troublesome, and in addition difficult to improve the accuracy thereof.